[AvocadosConstant]

I performed a back titration of Aspirin and NaOH, where the excess NaOH was titrated against HCl.
I crushed 1 tablet (1.20g, 0.3g of which was aspirin), in 250mL of 0.113M NaOH.
I then titrate 25mL samples of this solution, and the results show that an average of 20.8mL of 0.110M HCl is needed for the NaOH, subtracting the moles that reacted with HCl and moles of NaOH in 25mL gives 0.000537mols.
Given that NaOH and Aspirin reacted with a 1:1 ratio of mols, this equates to 0.0969g of Aspirin for 25mL and 0.969g per 250mL solution or 1 tablet. Given that I'm 0.669g off the expected value I am wondering where I might have gone wrong. Significant figures not large enough or lack of accuracy in procedure??

[Borek]

Hydrolysis?

[AvocadosConstant]

I thought that initially, but after researching the aspirin apparently takes a long time to hydrolyse, longer than the time it took to test it.

[AWK]

Most aspirin tablets are buffered. Look at the mass 1.2 g and 0.3 g aspirin content.